1. Technical Field
The present application relates to an optical read/write apparatus that reads and writes information from/on an optical storage medium such as an optical tape, an optical disc or an optical card.
2. Description of the Related Art
Recently, the size of digital data that can be stored on a storage medium has been rising steeply year by year as the resolutions of video data and still picture data have been tremendously increased and as increasing numbers of paper media have been converted into electronic ones.
Meanwhile, so-called “crowd computing” technologies that allow people to use various kinds of applications and services via servers and storage systems on some network have become more and more popular nowadays. According to such crowd computing technologies, as a huge number of users save various kinds of data on that storage system on the network, the amount of data accumulated there should keep on skyrocketing from now on.
In the meantime, as regulations have been established one after another with regard to the duty of preserving such a huge amount of data saved, it should also be increasingly important to devise a method for saving that enormous amount of data as securely and as reliably as possible.
As there is an increasing demand for saving such a huge amount of data with as much reliability as possible, people are lately considering using an optical pickup that forms a plurality of light beam spots on a single optical storage medium for an optical read/write apparatus, too. In that case, using those two light beam spots, two different sets of data can be read or written in parallel with each other or data can be written somewhere while another data is being read elsewhere. As a result, data can be read or written at an increased transfer rate or data being written can be read and verified simultaneously.
A known optical read/write apparatus that forms multiple light beam spots on an optical storage medium is disclosed in Japanese Laid-Open Patent Publication No. 63-249941.
FIG. 16A illustrates a simplified one of the optical pickup arrangement disclosed in Japanese Laid-Open Patent Publication No. 63-249941. The optical pickup is configured to perform read/write operations in parallel with each other using three light beam spots. Hereinafter, it will be described briefly how such an optical pickup works.
As shown in FIG. 16A, the optical system of the optical pickup includes a laser light source 1301, a diffractive element 1302, a beam splitter 1303, a collimator lens 1304, a wave plate 1305, an objective lens 1306, a condenser lens 1308 and a photodetector 1309. The light emitted from the laser light source 1301 gets diffracted by the diffractive element 1302 and split mainly into a zero-order light beam and ±first-order light beams, which are then condensed by the objective lens 1306, thereby forming three light beam spots (that are a main light beam spot 1311 and two sub-light beam spots 1312 and 1313) on the same track on the optical storage medium 1307.
FIG. 16B illustrates the arrangement of light beam spots that are formed on the surface of the optical storage medium 1307.
In the example illustrated in FIG. 16B, the main light beam spot 1311 formed by the zero-order light beam is a write beam spot, which is used to write a signal on the storage medium 1307. On the other hand, the two sub-light beam spots 1312 and 1313 formed by the ±first-order light beams are read beam spots, which are used to read a recorded track. Due to the efficiency ratio of the diffractive element, the intensities of the ±first-order light beams are much lower than that of the zero-order light beam.
The main light beam spot 1311 and the sub-light beam spots 1312 and 1313 are located on the same track. And these spots move on the storage medium in the direction indicated by the arrow 1314. Of these two sub-light beam spots, the sub-spot 1312 moves behind the main light beam spot 1311 for writing and reads the recorded mark. Meanwhile, the other sub-light beam spot 1313 moves ahead of the main light beam spot 1311 for writing, and its reflected light includes no information about the recorded mark. In FIG. 16B, the information storage layer of the optical storage medium 1307 has an uneven surface. The light beam spots move along a recording track 21 (i.e., a land portion, which may be a raised portion of the uneven surface) on which a mark will be recorded in the direction indicated by the arrow 1314. Actually, however, the light beam spots are fixed and the optical storage medium runs in the direction opposite to the one indicated by the arrow 1314. These light beams are reflected from the optical storage medium 1307, transmitted through the optical system, and then incident on the photodetector 1309, which detects their quantities of light.
FIG. 16C illustrates the arrangement of photodiodes in the photodetector 1309.
The quadruple photodiodes 1401 shown in FIG. 16C receive the zero-order light beam (i.e., the reflected light that has left the main light beam spot 1311 forms a detected light beam spot 1404 for writing). The magnitude of astigmatism produced by the condenser lens 1308 shown in FIG. 16A changes with the degree of defocusing, thereby detecting a focus signal. The photodiodes 1401 also detect a tracking error signal by the push-pull method.
On the other hand, the photodiodes 1402 and 1403 receive reflected light beams that have left the sub-light beam spots 1312 and 1313, respectively.
The laser light source 1301 emits a light beam that has been modulated by an optical modulator 1310 with a modulation signal in order to record a mark on the optical storage medium 1307.
Naturally, the read beams that have been emitted from the same laser light source and have left the sub-light beam spots 1312 and 1313 have also gone through that modulation. That is why the reflected light that has left the sub-light beam spot 1312 and that moves behind the main light beam spot for writing in the two read sub-light beam spots of the ±first-order light has a signal component, to which a variation in reflectance caused by a recorded mark and a variation in the quantity of light due to the modulation of light have been added. Meanwhile, the other sub-light beam spot 1313 moves ahead of the main light beam spot for writing through an unrecorded portion, and therefore, its reflected light has not been affected by the variation in reflectance caused by the recorded mark. Consequently, only a signal representing a variation in the quantity of light due to the modulation of the light by the light source is obtained from the reflected light that has left the sub-light beam spot that moves ahead of the main light beam spot.
For that reason, by performing a differential arithmetic operation on the two signals representing those two sub-light beam spots detected, a read signal (i.e., a monitor signal for verification purposes) can be obtained.
Thus, by forming the light beam spot for writing and the light beam spots for reading at the same time and by performing a verify operation by reading a signal that has just been written while performing a write operation, a system that achieves even higher write and transfer rate and ensures a good deal of reliability is realized.